In general, shovel-type construction machines such as hydraulic shovels and the like are configured such that an upper swing body is supported so as to be freely rotatable above a base carrier via a swing bearing; wherein, a swing frame, which serves as a base for said upper swing body, is provided with a bottom plate on which the swing bearing is mounted, a pair of left and right vertical frames (vertical plates) erected from said bottom plate for supporting an operation apparatus with which the upper swing body is equipped, and the like.
The swing bearing is attached to the bottom plate of the swing frame, as are a center joint for performing supply and drainage of hydraulic oil between the upper swing body and the base carrier, a swing motor apparatus for driving the swing bearing, and the like; however, the swing motor apparatus mounting portion of said bottom plate is a place where a large amount of stress is concentrated during rotation, wherefore it has been conventional practice to stack and weld a reinforcement plate on top of the bottom plate to obtain a doubling structure for reinforcement. When doing so, a seating surface for mounting the swing motor apparatus is formed on the reinforcement plate, that is, the reinforcement plate serves as a seat plate for mounting the swing motor apparatus; here, prior art configurations in which the reinforcement plate (seat plate for mounting the swing motor apparatus) is disposed extending in the left-right direction, and the left and right end portions of said reinforcement plate are welded to the inner side surfaces of the pair of left and right vertical plates erected on the bottom plate (e.g., Patent Document 1).
Further, in construction machines in which the swing motor apparatus is disposed further to the front side than the center joint, prior art configurations are also known in which the swing motor apparatus mounting portion is made as a thicker portion by making the plate thickness thereof thicker than that of the center joint mounting portion, and both the left and right end portions of said thicker portion are welded to the left and right vertical plates (e.g., Patent Document 2). In the construction machine described in Patent Document 2, a steel plate member having the plate thickness of the swing motor apparatus mounting portion is subjected to a milling process or the like on the portions thereof other than the swing motor apparatus mounting portion in order to remove material therefrom and make said portions thinner in plate thickness, whereby the swing motor apparatus mounting portion is made a thick portion.